benign forms of cancer
No there are not. And yes I am a doctor. Cancer is by definition NOT benign. Now you may be referring to less aggressive cancers and yes there are plenty of those. Basal cell carcinoma for example. Some prostate cancers. Cervical cancers. All of those are very slow in the growing and spreading. However thyroid cancers, due to their location, are obvious pretty quickly. People tend to wonder about that lump sticking out of their throat.
Still all of that is besides the point. When autopsies are performed, any cancers are noted even when the patient dies of unrelated conditions. For example almost ALL men over age 80 and ALL men above 90 have prostate cancer, although most of them die from something else. So yes you're right in that cancer can go undetected. But you are wrong in thinking we don't know exactly what the "normal" amount of cancer is in a population.